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interferring with functionity

Originally Posted by mrhoo

a bad idea

That's an under-statement. Users come to expect certain behaviour from certain page elements. Reset buttons reset, Submit buttons submit, Text inputs accept a word or single line of text, and textareas accept multiple lines of text. Even with a alert, stubborn users might "copy paste" the cr, but most will leave and not return. I have the feeling you taking the wrong approach to solving a problem here.
Perhaps if you explain why you don't want the textareas to behave normally a better solution could be found.